1. Field of the Invention
The present invention relates to a power supply apparatus that supplies a bus voltage to a device and a bus controller employed in the power supply apparatus.
2. Description of the Related Art
In recent years, various kinds of electronic devices (which will be simply referred to as the “devices” hereafter) such as smartphones, tablet terminals, laptop personal computers, portable audio players, digital still cameras, digital video cameras, and the like, are configured to conform to the USB (Universal Serial Bus) standard or the like, which allows electric power to be supplied via a cable.
In particular, in addition to a chargeable secondary battery, a battery-driven device includes a charger circuit in order to charge the secondary battery. Known examples of such charger circuits include an arrangement that charges such a secondary battery using a DC voltage supplied from a USB (Universal Serial Bus) host adapter via a USB cable.
At present, charger circuits to be mounted on mobile devices conform to a specification which is referred to as the “USB Battery Charging Specification” (which will be referred to as the “BC specification” hereafter). There are several kinds of host adapters. In revision 1.2 of the BC specification, SDP (Standard Downstream Port), DCP (Dedicated Charging Port), and CDP (Charging Downstream Port) have been defined as the kinds of chargers. The current (current capacity) that can be supplied by a host adapter is determined according to the kind of charger. Specifically, DCP and CDP are defined to provide a current capacity of 1500 mA. Also, SDP is defined to provide a current capacity of 100 mA, 500 mA, or 900 mA, according to the USB version.
As a next-generation secondary battery charging method using USB, a specification which is referred to as the “USB Power Delivery Specification” (which will be referred to as the “PD specification” hereafter) has been developed. The PD specification allows the available power to be dramatically increased up to a maximum of 100 W, as compared with the BC standard, which provides a power capacity of 7.5 W. Specifically, the PD specification allows a bus voltage VBUS that is higher than 5 V (specifically, 12 V or 20 V). Furthermore, the PD specification allows a charging current that is greater than that defined by the BC specification (specifically, the PD specification allows a charging current of 2 A, 3 A or 5 A).
A host adapter that supplies electric power to a device includes a DC/DC converter that generates a DC voltage used as a bus voltage. In recent years, from the viewpoint of power saving, there is a demand for a power supply apparatus having power consumption that is further reduced as much as possible when it operates with a light load or otherwise operates without a load (which will be referred to as the “standby state”). In order to meet this demand, a DC/DC converter operates in a so-called burst mode (which will also be referred to as the “PFM mode”) in the standby state. In the burst mode, a switching transistor is switched on and off once or otherwise several times such that an output voltage becomes greater than a target level, and suspends the switching of the switching transistor until the output voltage drops to a lower limit level determined according to the target level of the output voltage. Such an arrangement reduces the electric power used to switch on and off the switching transistor (the electric power required to charge and discharge the gate capacitance of the switching transistor, for example), thereby providing improved efficiency.
However, there appears to be no end to the demand for reducing power consumption, and so such a power supply apparatus is required to provide further reduced power consumption.